Activity-regulated changes in synapse function lie at the heart of molecular theories of learning and neural development, and are the targets of diseases and disorders including Alzheimer's Disease, schizophrenia, and epilepsy. At glutamatergic synapses of the brain, multi-domain proteins establish the core of the postsynaptic density (PSD), the structure which links neurotransmitter receptors to the actin cytoskeleton and to intracellular signaling pathways. It has been proposed that such PSD proteins control synaptic strength by controlling the anchoring or mobility of synaptic receptors, but direct evidence regarding how this might be accomplished within the PSD has been lacking. Furthermore, though PSD size and shape correlate with synaptic receptor levels when assayed as a population at single time points, it is not known whether single PSDs transition among various morphologies, or whether such transformations mediate weakening of synapses during long-term depression. We have established a novel fluorescence morphometry approach for examining single, living PSDs, and have developed two high-resolution photobleaching assays for monitoring the movement of proteins within them. We have found that PSDs undergo a large degree of continuous morphological change, contrary to expectations for a simple scaffold. However, mobility of core protein within the complex is extremely limited, indicating that internal structure is nevertheless maintained. We therefore hypothesize that dynamic adjustment of PSD form continuously alters synapse function, and that long-term synaptic depression requires the disruption of this structure to accommodate the removal of receptors. In this proposal, we will clarify the role of PSD internal dynamics in synaptic function and plasticity. In cultured neurons from rat hippocampus, we will use quantitative fluorescence microscopy along with patch-clamp electrophysiology, photolysis, and time-resolved electron microscopy to address the following specific aims. First, is PSD morphological change coordinated with presynaptic structure? Second, do these changes dynamically alter synaptic strength? Third, does actin control the internal stability of the postsynaptic scaffold? Fourth, does the PSD undergo regulated changes in morphology or dynamics before or after receptor loss in long-term depression? The answers to these questions will fill large gaps in our understanding of the synaptic structure-function relationship. Further, they will provide an important platform on which to test hypotheses regarding the molecular basis of disorders that disrupt synaptic transmission.